The present invention relates to a heating and air-conditioning installation for motor vehicles, by means of which an adjustable temperature is kept constant in the interior space of a motor vehicle independent of changing conditions. The installation includes a heat-exchanger that is connected to the hot water circulation system of the engine, as well as with an adjusting member, on the position of which is dependent the heat quantity supplied to the vehicle interior space. The position of the adjusting member is controlled by an output signal produced by an electronic circuit in dependence on a sensing device detecting the heat quantity supplied to the vehicle interior space, on an interior temperature-sensing device (interior sensor), on a setting means for setting the desired value and on an outside temperature-sensing device (outside sensor). A fan and a switching device for switching from fresh-air operation to recirculated-air operation are also controlled in dependence on a signal of the electronic circuit.
It is known in the prior art to control the temperature in the interior space of a motor vehicle with the aid of an electronic circuit which has, inter alia, the task of acting upon an adjusting member that controls the supply of more or less heated air to the vehicle interior space. This control operation takes place in dependence on value which are fed to the electronic circuit from a sensing device detecting the heat quantity supplied to the vehicle interior space, a temperature-sensing device which detects the interior temperature in the vehicle (interior sensor) as well as a temperature value adjustable at a setting means in the interior of the motor vehicle. In a known type of construction the difference between the values of the sensing device detecting the supplied heat quantity and the interior sensor is used as a guide magnitude during this control operation. The control deviation is then formed from the difference of this difference with the desired value.
It has been proved as disadvantageous for small changes in the adjustment of the desired value potentiometer to lead to a sudden response of the heating or air-conditioning system so that pressure and temperature changes in the interior space of the vehicle which also occur suddenly as a result thereof, impair the driving comfort. Additionally, the prior art air-conditioning installation also responds to brief temperature changes in the vehicle interior space, i.e., temperature changes which occur over short periods of time, for example, during a brief opening of the window, which is neither necessary nor desirable. It is already known to detect the temperature on the outside air of the vehicle with the aid of an outside sensing device for influencing the proportionality range of this control cycle, whereby, however, the rapid undesired response in case of small temperature changes in the vehicle interior space can not be completely eliminated.
A further disadvantage of the prior art air-conditioning installation resides in the manner of the switching or shifting from fresh-air to recirculated-air operation. Such a switching will always be undertaken when the evaporator of the air-conditioning installation has reached the limits of its output capacity. This is the case in particular at very high outside temperatures and at the same time at very high relative air humidities, whereby the air-conditioning installation then no longer branches off the air required for cooling from the outside air, but instead is connected with the vehicle interior space by way of a duct so that already pre-cooled air is used for the further cooling. This switching over from fresh-air operation to recirculated-air operation takes place in the prior art air-conditioning systems when the difference between the set or desired value and the instantaneous existing value detected at the interior sensing device is used as guide magnitude for this switching operation. This has a consequence that the installation switches over in regular intervals from fresh-air to recirculated-air and vice-versa, whereby a constant actuation of the shifting mechanism is required. As a result thereof, the pressure as well as the noise conditions also change constantly in the interior space of the vehicle.